Auditory brainstem neurons fire at very high rates with extraordinarily high temporal precision, allowing them to encode specific features of sound stimuli. Unexpectedly, it has been found that the levels and characteristics of potassium channels in these neurons are rapidly modified by the auditory environment. We plan to determine the mechanisms of this use-dependent plasticity for two classes of K+ channels, i) voltage-dependent Kv3.1b channels and ii) Na+-activated K+ channels (KNa channels) encoded by the Slack and Slick genes. The function of Kv3.1b channels is to allow neurons to fire at high frequencies. We plan to determine whether sound- induced increases in Kv3.1b protein levels result in higher Kv3.1b currents in the plasma membrane and enhance the ability of MNTB neurons to fire at higher rates. We also plan to test the hypothesis that KNa current amplitude is regulated by sound-induced changes in phosphorylation state to enhance the temporal accuracy of these neurons at high rates of stimulation. For both Kv3.1b and KNa channels we will determine whether the activity-dependent increases in current are regulated by the Fragile X Mental Retardation Protein (FMRP), a repressor of protein translation that binds Kv3.1 mRNA and that modulates KNa channels by direct protein- protein interactions. An understanding of how the excitability of auditory neurons is regulated by physiological stimuli is likely to lead to novel pharmacological treatments for disorders of auditory function including tinnitus, age-related hearing loss and audiogenic seizures, as well as Fragile X syndrome and other disorders of excitability.